Database Migration

Note: The database migration feature has been available since version 1.1.6.

Like source code, the structure of a database is evolving as we develop and maintain a database-driven application. For example, during development, we may want to add a new table; or after the application is put into production, we may realize the need of adding an index on a column. It is important to keep track of these structural database changes (called migration) like we do with our source code. If the source code and the database are out of sync, it is very likely the whole system may break. For this reason, Yii provides a database migration tool that can keep track of database migration history, apply new migrations, or revert existing ones.

The following steps show how we can use database migration during development:

Tim creates a new migration (e.g. create a new table)

Tim commits the new migration into source control system (e.g. SVN, GIT)

Doug updates from source control system and receives the new migration

Doug applies the migration to his local development database

Yii supports database migration via the yiic migrate command line tool. This tool supports creating new migrations, applying/reverting/redoing migrations, and showing migration history and new migrations.

In the following, we will describe how to use this tool.

Note: It's better to use application-specific yiic (e.g. cd path/to/protected)
when working with migrate command instead of one from framework directory.
Make sure you have protected\migrations directory and it is writable. Also check if
you've configured database connection in your protected/config/console.php.

To create a new migration (e.g. create a news table), we run the following command:

yiic migrate create <name>

The required name parameter specifies a very brief description of the migration (e.g. create_news_table). As we will show in the following, the name parameter is used as part of a PHP class name. Therefore, it should only contain letters, digits and/or underscore characters.

yiic migrate create create_news_table

The above command will create under the protected/migrations directory a new file named m101129_185401_create_news_table.php which contains the following initial code:

Notice that the class name is the same as the file name which is of the pattern m<timestamp>_<name>, where <timestamp> refers to the UTC timestamp (in the format of yymmdd_hhmmss) when the migration is created, and <name> is taken from the command's name parameter.

The up() method should contain the code implementing the actual database migration, while the down() method may contain the code reverting what is done in up().

Sometimes, it is impossible to implement down(). For example, if we delete table rows in up(), we will not be able to recover them in down(). In this case, the migration is called irreversible, meaning we cannot roll back to a previous state of the database. In the above generated code, the down() method returns false to indicate that the migration cannot be reverted.

Info: Starting from version 1.1.7, if the up() or down() method returns
false, all the following migrations will be canceled. Previously in version
1.1.6, one has to throw exceptions to cancel the following migrations.

Info: You may notice that the database methods provided by CDbMigration are very similar to those in CDbCommand. Indeed they are nearly the same except that CDbMigration methods will measure the time used by their methods and print some messages about the method parameters.

Info: The feature of transactional migrations has been supported since version 1.1.7.

While performing complex DB migrations, we usually want to make sure that each migration succeed or fail as a whole so that the database maintains the consistency and integrity. In order to achieve this goal, we can exploit DB transactions.

We could explicitly start a DB transaction and enclose the rest of the DB-related code within the transaction, like the following:

When Yii performs the migration, it will start a DB transaction and then call safeUp() or safeDown(). If any DB error occurs in safeUp() or safeDown(), the transaction will be rolled back, thus ensuring the database remain in a good shape.

Note: Not all DBMS support transactions. And some DB queries cannot be put
into a transaction. In this case, you will have to implement up() and
down(), instead. And for MySQL and MariaDB, some SQL statements may cause implicit commit
(see documentation of MySQL and
MariaDB for details).

To apply all available new migrations (i.e., make the local database up-to-date), run the following command:

yiic migrate

The command will show the list of all new migrations. If you confirm to apply the migrations, it will run the up() method in every new migration class, one after another, in the order of the timestamp value in the class name.

After applying a migration, the migration tool will keep a record in a database table named tbl_migration. This allows the tool to identify which migrations have been applied and which are not. If the tbl_migration table does not exist, the tool will automatically create it in the database specified by the db application component.

Sometimes, we may only want to apply one or a few new migrations. We can use the following command:

yiic migrate up 3

This command will apply the 3 new migrations. Changing the value 3 will allow us to change the number of migrations to be applied.

We can also migrate the database to a specific version with the following command:

yiic migrate to 101129_185401

That is, we use the timestamp part of a migration name to specify the version that we want to migrate the database to. If there are multiple migrations between the last applied migration and the specified migration, all these migrations will be applied. If the specified migration has been applied before, then all migrations applied after it will be reverted (to be described in the next section).

Sometimes, we may want to modify the migration history to a specific migration version without actually applying or reverting the relevant migrations. This often happens when developing a new migration. We can use the following command to achieve this goal.

yiic migrate mark 101129_185401

This command is very similar to yiic migrate to command, except that it only modifies the migration history table to the specified version without applying or reverting the migrations.

Use Command Line Options

The migration command comes with four options that can be specified in command line:

interactive: boolean, specifies whether to perform migrations in an interactive mode. Defaults to true, meaning the user will be prompted when performing a specific migration. You may set this to false should the migrations be done in a background process.

migrationPath: string, specifies the directory storing all migration class files. This must be specified in terms of a path alias, and the corresponding directory must exist. If not specified, it will use the migrations sub-directory under the application base path.

migrationTable: string, specifies the name of the database table for storing migration history information. It defaults to tbl_migration. The table structure is version varchar(255) primary key, apply_time integer.

connectionID: string, specifies the ID of the database application component. Defaults to 'db'.

templateFile: string, specifies the path of the file to be served as the code template for generating the migration classes. This must be specified in terms of a path alias (e.g. application.migrations.template). If not set, an internal template will be used. Inside the template, the token {ClassName} will be replaced with the actual migration class name.

To specify these options, execute the migrate command using the following format

yiic migrate up --option1=value1 --option2=value2 ...

For example, if we want to migrate for a forum module whose migration files are located within the module's migrations directory, we can use the following command:

yiic migrate up --migrationPath=ext.forum.migrations

Note that in case you're setting boolean options such as interactive using command line you should pass either 1
or 0 like the following:

yiic migrate --interactive=0

Configure Command Globally

While command line options allow us to configure the migration command on-the-fly, sometimes we may want to configure the command once for all. For example, we may want to use a different table to store the migration history, or we may want to use a customized migration template. We can do so by modifying the console application's configuration file like the following,